Microphones serve to convert sound signals into corresponding electrical ac voltage signal. Those ac voltage signals are referred to as microphone signals. The microphone signals are transmitted from the microphone by way of lines for example to amplifiers or mixer desks.
To provide the power supply for microphones a direct current can be transmitted in the opposite direction to the microphone signal. For that purpose a dc voltage is applied to the same lines as those on which the ac voltage signal is transmitted from the microphone. In the case of capacitor microphones such a power supply is referred in audio technology as the phantom feed. The power supply for the capacitor microphones serves inter alia to drive the impedance converter in the microphone. It can also be used for polarization of the capacitor converter contained in the microphone.
FIG. 1 shows a diagrammatic circuit illustrating the principle of a microphone having a phantom feed in accordance with DIN 61938 (up to 1979 DIN 45596). In that respect FIG. 1 shows a feed portion 1 connected to a microphone 2 by way of a line 3. The line 3 has at least two line wires a and b, on which the ac voltage signal produced by the microphone 2 is transmitted to the feed portion 1, as well as a screen c also connected to the feed portion 1 and the microphone 2. The ac voltage signal is made available for example for further use at the feed portion 1, by way of the audio signal outputs a and b. The feed portion 1 can be connected to ground by way of a grounding terminal c.
In addition in the feed portion 1 there is a dc voltage source 11 which provides a feed voltage U. In that respect one pole of the dc voltage source 11 is connected to the screen c and the other pole of the dc voltage source 11 is connected to two resistors R1 and R2 arranged in mutually parallel relationship. The resistors R1 and R2 are respectively connected to one of the two line wires a and b. The audio signal outputs a and b at the feed portion 1 are connected directly to the terminals of the line wires a and b and the two resistors R1 and R2. The resistors R1 and R2 are also referred to as feed resistors.
The two resistors R1 and R2 have ohmic resistances which are as identical as possible. In that way the operating current I flows in respective halves thereof from the dc voltage source 11 of the feed portion 1 by way of the two resistors R1 and R2 and the line wires a and b of the line 3 to the microphone 2. In that case there is an identical dc voltage in relation to the screen c at the line wires a and b, and for that reason direct current transmission on the line wires a and b does not influence the ac voltage signal transmission of the microphone signal. The resistors R1 and R2 should have resistances which are as far as possible of the same value in order to ensure low-interference signal conduction by circuitry symmetry. In addition the resistors R1 and R2 serve for decoupling the audio signal generated by the microphone 2 from the dc voltage source.
A number of examples of a phantom feed are set out as standards in DIN 61938. In that respect, the feed voltages U, feed currents I and feed resistors R1 and R2 are established in accordance with Table 1 for the basic circuitry shown in FIG. 1. In practice the feed voltage of 48 V (phantom feed P 48) is widespread.
TABLE 1Standard values of the phantom feedin accordance with DIN 61938Feed voltage U12 ± 1V24 ± 4V48 ± 4VFeed current Imax. 15mAmax. 10mAmax. 10mAR1 and R2680Ω1.2Ω6.8kΩ
The diagrammatic basic circuit shown in FIG. 1 suffers from the disadvantage that the operating current of the microphone 2 at the feed resistors R1 and R2 leads to a power loss which has to be additionally applied by the feed voltages 11 and which is converted into heat in the feed resistors R1 and R2.
That becomes clear in particular in the case of a phantom feed involving 48 V if the microphone 2 consumes the maximum operating current of 10 mA, which is permissible in accordance with the standard. Then the total power to be applied in the basic circuit shown in FIG. 1 by the feed voltage source 11 is 0.48 W. Of that, 0.14 W is consumed by the microphone 2 while 0.34 W is converted into heat in the feed resistors R1 and R2. The ratio of useable power in the microphone 2 to the total power, that is to say the level of efficiency, is in that case only about 30%, that is to say the power supply must be oversized by a factor of about 3. The power to be additionally applied can considerably add up if a plurality of microphones 2 have to supplied at the same time. Also if the supply is from batteries or accumulators, the operating time is considerably reduced.